An apparatus for filtration is well-known in which foreign matter contained in liquid is removed by adhering to fiber of filter paper while the liquid passes through filtration passages which are gaps between the layers of the filter paper.
When incorporating such filtration apparatus in a circuit for filtration of lubricating oil for an engine used in an automobile or a ship, the filter element is placed under high pressure at the time of starting or rapid acceleration of the engine or at a time when a by-pass circuit is out of order. As a result, the filter element is partially crushed under pressure and gets wrinkled, so that a leak passage for the liquid is created in a part of the filter element.
In comparison with the size of the impurities, the thus formed leak passage is an extremely gigantic passage, through which the impurities can freely pass, resulting in proper filtration being made impossible.
Furthermore, in the filtration circuit for lubricating oil for an engine used in an automobile or a ship, air leaks into the circuit when the engine stops. Thus, because air leaks into the filtration circuit the lubricating oil is recontaminated when the air turns into foam with a rise in pressure in the circuit and the foam mixes in the lubricating oil and passes through gaps between the layers of the filter element, thereby driving away and setting free sludge which has been removed from the liquid adherence to the fiber of the filter element.